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Structure and ultrastructure of the frog motor endplate
Authors:K. Peper  F. Dreyer  C. Sandri  K. Akert  H. Moor
Affiliation:(1) II. Physiologisches Institut der Universität des Saarlandes, D-665 Homburg/Saar, Germany;(2) Brain Research Institute, University of Zürich, Switzerland;(3) Laboratory of Electron microscopy, Department of General Botany, Federal Institute of Technology, Zürich, Switzerland
Abstract:Summary The frog motor endplate in its simplest form consists of an elongated, slender nerve ending embedded in a gutter-like depression of the sarcolemma. This nerve terminal contains the usual synaptic organelles. It is covered by a thin coating of Schwann cell cytoplasm which embraces the terminal with thin finger-like processes from both sides, thereby sub-dividing it into 300–1000 regularly spaced compartments. The individual synaptic compartments correspond to the strings of varicosities or grape-like configurations of motor nerve terminals in endplates of other species and in the cerebral neuropil of vertebrates.Each compartment contains one or more bar-like densities of the presynaptic membrane, lsquoactive zonesrsquo, which are associated with the attachment sites between synaptic vesicles and plasmalemma. Active zones have a regular transverse arrangement and occur at specific loci opposite the junctional folds. The attachment sites for synaptic vesicles are at the edges of the bars which are bilaterally delineated by a double row of 10 nm particles attached to the A-face. The structural appearance of vesicle attachment sites in freeze-etch replicas corresponds to that of micropinocytosis. The active zones are often fragmented and the frequency of their association with vesicle attachment sites is highly variable.The junctional folds are characterized by ldquospecific sitesrdquo in which intramembranous particle aggregations occur at relatively high packing density (7500/mgrm2). These sites are located opposite the active zones at the juxtaneural lips, a location where one would expect ACh-sensitive receptors on the postsynaptic membrane.This work was supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 38, Projekt N), The Swiss National Foundation for Scientific Research (Grants Nr. 3 82372 and 3 77472) and the Dr. Eric Slack-Gyr Foundation Zürich.
Keywords:Motor endplate  Rana esculenta  Ultrastructure and function  Freeze-etching electron microscopy
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